1. Field of the Invention
The present invention relates to the production of phase shifting masks for extreme ultraviolet lithography (EUVL), and more specifically, it relates to systems and methods for directly writing patterns into the reflective multilayer coating of an extreme ultraviolet lithography phase shifting mask.
2. Description of Related Art
Phase shifting masks give rise to images of much improved contrast as compared with conventional binary masks used in microlithography. In extreme ultraviolet lithography (EUVL), masks are flat substrates with a reflective multilayer coating. Phase-shift EUV masks have been proposed by adding material to the top surface of the multilayer. The material is chosen for its property to phase shift a beam transmitting through it without significantly absorbing. Even so, all materials will absorb, and so the phase shift is accompanied by an undesirable absorption.
The use of phase-shifting masks is a resolution-enhancing technique that is well known in optical microlithography. Its use has also been proposed for EUV lithography. In conventional lithography, where phase shifting is not used, the mask is binary and consists of either reflective or absorbing regions to define the pattern to be printed. The pattern is imaged onto the wafer by the projection optics. For patterns with feature sizes approaching the resolution limit of the projection optics the contrast of the image is much reduced as compared with the contrast (essentially unity) of the mask pattern. The contrast reduction is due somewhat to aberrations of the projection optics, but mostly (in high quality lithographic systems) due to the finite numerical aperture of the projection optics. For example, the aerial image intensity might vary between 0.6 and 0.3 of the bright-field intensity in the bright and dark areas of a pattern, respectively. This would lead to a contrast of 0.3. The low contrast leads to a reduced tolerance to changes in dose or focus in the printing of acceptable patterns into photoresist, or may lead to unacceptable printed patterns even at best dose and focus. The use of phase shifting elements in the mask can dramatically enhance the image contrast. For example, consider a feature on the mask consisting of two adjacent regions that are bright (non-absorbing) such that one region imparts a phase change of π to the reflected beam relative to its neighboring region. The complex amplitude of the reflected beam then varies between 1 and −1. The image of this feature will always pass through zero amplitude (and hence zero intensity) at the boundary of the regions, no matter what aberrations are present in the projection optics. That is, the dark region in the image is zero intensity and so the image contrast will be unity.
Masks for EUV lithography consist of flat multilayer-coated substrates and, in standard operation, are patterned with overlying absorbing features. One method that has been proposed for achieving a phase change on the mask is by adding material to the top of the multilayer. The phase of the light that passes through this material, to be reflected from the multilayer, and traverse the material once more is 4π(n−1)t/λ, where n is the refractive index of the material, t is the thickness of material, and λ is the wavelength of EUV light. However, the disadvantage of this technique is that, for any material chosen, the required thickness of material to cause the desired π phase shift will also cause absorption of the EUV light. For example, a 42.9 nm thick layer of Mo will cause a π phase shift to the reflected 13.4 nm wavelength beam and a corresponding reduction in reflected intensity by a factor of 0.6.